1. Field of the Invention
This invention relates broadly to tension leg platforms for offshore production and drilling, and more particularly, to an apparatus and method for compensating for undesirable changes in the tension loading of tension legs used to moor such platforms to the sea floor.
2. Description of the Prior Art
The following statement is intended to be a prior art statement in compliance with the guidance and requirements of 37 C.F.R. .sctn..sctn.1.56, 1.97 and 1.98. In the exploration and production of hydrocarbons from a subsea formation, problems of weight and expense are encountered in very deep drilling and production activities which render the use of bottom-founded steel or concrete supporting structures less than optimum, and in some cases prohibitive. It is more economical to provide a semi-permanent site for producing and drilling operations in deep water by using a floating platform which is moored or tethered to anchor points on the sea floor, using vertical tension legs to moor the platform above the drilling or production sites. Such an assembly is known as a tension leg platform.
The use of pretensioned mooring legs prevents vertical motion or heave of the platform during wave passage, yet permits lateral deflection of the entire assembly. Leg pretensioning is accomplished by deballasting the floating platform after the tension legs have been connected to the sea floor anchor points. Such pretensioning prevents the tension legs from becoming slack during the passage of the troughs of most waves associated with even extreme environmental conditions.
After a tension leg platform has been constructed and the tension legs pretensioned by deballasting the platform, certain conditions can arise over the life of the structure which severely impair its usefulness and constitute possible extreme hazards to not only the drilling or production operation, but to the safety of the personnel on the platform. Unless the sea floor anchor foundations to which the tension legs are connected are positioned extremely accurately during construction of the platform, the pretension in the several tension legs will vary from leg to leg, causing possible overstressing of one of the legs as continuing wave action acts on the platform.
A more serious concern is that which is posed by the possibility of severe hurricane or cyclonic storm conditions which may generate giant waves at the locale of the tension leg platform. On such an occasion, the trough of such a giant wave will develop a slacked tether condition in which one or more of the tension legs is slacked and thus can collapse under its own weight. This condition is aggravated where the anchor foundations on the sea floor to which the tension legs are attached have been to any extent mispositioned. Moreover, even should the tension legs not collapse in the described slack tether condition, the following wave crest may suddently restore an over-tensioned condition to one or more of the tension legs, tending to crack or pop them similarly to a whip, with immediate structural failure.
U.S. Pat. No. 3,983,706 to Kalinowski is directed to improvements in one type of tension cable offshore platform structure, such improvements residing in the ability to hydraulically tension and realign a vertical riser extending from the wellhead to the floating drilling platform. In order to compensate for the deflection of the riser from a vertical position under the impress of subsurface currents, or due to shifting of the floating platform in heavy seas, a plurality of hydraulic piston and cylinder assemblies are extended between the vertical riser and a plurality of tension cables spaced around the riser and connected between peripheral points of the platform and anchor blocks secured to the floor of the sea. Control of the hydraulic cylinders so as to compensate for positional shifting of the riser is accomplished from the floor of the platform by hydraulic conduits extended down along the side of the riser to the piston and cylinder assemblies. The structure described in the Kalinowski patent is not concerned with compensating for tension in the flexible tension cables used to moor the floating drilling platform depicted and described in that patent, and in fact there is no disclosure of any means for making any vertical adjustment in the relative positions of the floating platform and the upper portion tension legs in order to compensate for a slack tether condition resulting from an excessive wave troughing condition.
Another tension cable supported floating platform is illustrated and described in Engle, Jr. et al U.S. Pat. No. 4,114,393. The Engle patent is directed to an improvement in such platforms which damps the tension cables by interconnecting them at certain selected points so as to prevent resonant fluttering of the cables at certain flutter frequencies likely to be encountered, thus increasing the useful life of the cables. This structure, of course, experiences problems and considerations differing from tension leg platforms which employ tension legs formed by interconnected rigid tubular sections extended from anchor points to the platform, and pretensioned by deballasting of the platform.
Hydraulic jacks have been employed for aiding in extending the life of the support legs used in another type of offshore drilling platform called a jack-up rig. In these rigs, the platform is actually elevated above the surface of the ocean by a jacking action which extends the legs vertically during installation of the rig. With rigs of this type, problems arise from the severe shock forces to which the drilling rigs are subjected when they are placed upon or taken off of the ocean floor. This is due to the subjection of the platform at this time to forces tending to shift or move it and lift it up or down due to wave and current action, with the relatively stiff supporting legs then being subjected to sudden compressive loading and consequent damage. In U.S. Pat. No. 4,195,950, it is proposed to provide a shock-absorbing structure to be mounted on the bottom of each of the platform-supporting legs, utilizing hydraulic jacks at this location and associated compression members which surround the piston elements of the jacks so that such compression members absorb the shocks which would otherwise be transmitted directly to the legs during severe conditions at the rig location.
In United Kingdom Patent Application No. 2,035,240A filed on Nov. 14, 1979, a tether assembly for a tethered buoyant offshore platform is described. Hydraulic jacks are provided on the platform for pretensioning the tether shafts employed to moor the platform to the sea floor. After this time adjustments in the tension loading of the tether shafts is achieved primarily by shims. Some further adjustment in tether tension and also in tether length is achieved mechanically by the use of tether length adjustors, and also by hydraulic jacks which can be connected to the upper ends of the tether shafts by cables or a make-up piece. No arrangement is provided for automatically tensioning the tether shafts to compensate for an approach to a slack tether condition induced by extreme weather conditions.